1. Field of Invention
The present invention relates generally to communications antennas and in particular to a UHF mobile communications antenna that may be attached to a vehicle window without drilling into or damaging the surface of the vehicle or window.
2. Description of the Related Technology
Mobile two-way communications for police, fire, taxicab and business use have generally been in the radio frequency bands of 32-40 MHz, 150-174 MHz, 420-270 MHz and 800-900 MHz. Cellular telephones using the 800 MHz band have become extremely popular for both business and personal use in recent years. As the radio bands used increase in frequency, higher gain and more efficient antennas are desirable. In addition to high efficiency and high gain, a broad bandwidth is also necessary for mobile telephone applications such as, for example, the new cellular mobile systems. Desirable features in any antenna system are broad bandwidth, gain, good efficiency, and low voltage standing wave ratio ("VSWR"). These requirements are necessary for proper operation with modern day mobile radio transceivers.
At VHF and UHF radio frequencies the distance that one may communicate is normally limited to line of sight. Therefore, the higher the mobile antenna is located on the vehicle, the better the useful communications range. A desirable antenna radiation pattern will produce the most efficient operation of the antenna and result in maximum clear signal range. The radiation pattern of an antenna is affected by the antenna's proximity to metal objects on or of the vehicle. Therefore, the most desirable location for an antenna on a vehicle would be the rooftop or mounting as close to the roof line of an automobile as possible.
To permanently mount a mobile antenna on a vehicle required drilling or cutting a hole into the vehicle body and then inserting the antenna in the hole. The metal surface of the vehicle acted as a ground plane for the mounted vertical antenna radiating element. Use of the vehicle body as a ground plane was mandatory for maximum efficiency in antenna operation. With alternate locations on a car such as the fender or trunk lid or bumper, the ground plane was not optimal and the antenna efficiency and radiation pattern suffered.
The hole cut in the vehicle body for installation of the mobile antenna resulted in damage that was expensive to repair when the antenna had to be removed. As automobiles increased in price and the cost of subsequent body repair work increased, a more satisfactory arrangement for permanently mounting mobile radio antennas to expensive vehicles without damage was desired without sacrificing communications performance.
Antenna systems, called "on-the-glass antennas," were devised that were removably mountable on a window of a vehicle without damage thereto. An antenna part was attached to the exterior surface of the window by means of glue, such as epoxy, or adhesive tape pads. An interior part was attached to the interior surface of the same window by similar means and in alignment with the antenna part. Radio frequency energy was transferred between the interior part and exterior antenna part by capacitive coupling.
A coupling capacitor was created when a conductive base plate, connected to the antenna element, was mounted to the exterior surface of a window forming one side of the capacitor, and an interior plate was mounted to the interior of the window opposite the exterior plate forming the other plate of the capacitor. The window glass was the dielectric insulator between the exterior and interior plates. Thus, radio frequency signals could pass between the exterior and interior parts of the antenna system without the necessity for any actual physical connection requiring a hole in the vehicle.
Early on-the-glass antennas, however, suffered from ineffective and inefficient ground planes or ground counterpoises. These antennas used either the interior metal parts of the vehicle or the coaxial cable outer shield conductor as a ground counterpoise. Using either resulted in radiation of radio frequency energy inside of the passenger compartment. This undesirable radiation was emitted from the outer shield of the coaxial cable that connected the antenna system to a mobile radio transceiver. Studies have revealed that radiation of radio frequency energy from a mobile radio transceiver may cause harmful effects to occupants within a vehicle. Therefore, reduction of radio frequency radiation within the vehicle was very important.
Further work was needed to reduce this radio frequency radiation problem. An early attempt to solve the radiation problem was to utilize a coupling device attached to the inside window that would function as a ground counterpoise and effectively decouple stray radio frequency energy from the outer coaxial cable shield. In addition, matching networks of various types were employed to reduce the VSWR of these antenna systems. A device of this type is shown in U.S. Pat. No. 4,839,660, the disclosure of which is incorporated by reference herein. However, all of the aforementioned on-the-glass antenna systems used capacitive (electrostatic coupling) to couple the radio frequency energy between the interior part and the exterior antenna part.
Capacitive coupling was detrimentally affected by stray capacitive coupling from adjacent conductive metal parts of the vehicle such as the window trim ring or embedded de-icing strips in a rear window. Normally, because of the vagaries of mounting locations for on-the-glass mobile antennas, the capacitive coupling effects of each installation cannot be determined beforehand with any certainty. Adjustable matching networks have been incorporated into antenna systems to tune for optimal performance after installation. Thus, each type of vehicle and different location thereon required post installation tuning adjustment for proper operation. Tuning of the antenna matching system required special test equipment and special knowledge of the radio technician to effectively adjust the antenna system for maximum radiation and minimum VSWR.